The present invention relates generally to digital signal detection circuits and, more particularly, to a digital signal detector and method for detecting the presence or absence of digital information on a signal path, such as a telephone line, wherein digital code samples from past sample periods are compared with a digital code sample from an instant sample period. If no change has occurred over one or more sample periods, the signal path is idle and no digital information is being transmitted on the signal path.
Numerous devices associated with the telephone industry rely upon detection of whether a telephone line is connected to a live calling circuit or to an inactive circuit. In other words, whether a telephone line is currently being used. For example, automatic call distributors (ACDs) perform various functions which are dependent upon the detection of signals on a telephone line, such as answer detection and the like.
Prior systems utilize analog automatic gain control (AGC) or digital signal line processor circuits which compare detected signal levels with a preset, or monitored background, level. When the detected signal level drops below the preset level for predetermined amount of time, the telephone line is considered idle. Conversely, if the detected signal level exceeds the preset level, the telephone line is considered active (i.e. is transmitting information). These methods are influenced by the amount of digitized noise present. For example, if the preset level is set below the background noise level of the telephone signal, the prior methods would never indicate that the telephone line is idle. Additionally, if the message information is buried amongst background noise, the actual signal presence may never be detected. Background noise, as is well known, may also be generated during the digitization process by the detector amplifier or by the detector circuit. And, as is also well known, the amount of background noise typically changes over time as the electronic circuitry ages.
Accordingly, there is a need in the art for a signal detector and method for detecting digital information on a telephone line which is substantially independent of the amount of noise present on the telephone line at any given time, which provides relatively accurate results and which is relatively simple to construct.